The present invention relates generally to surface cleaning and, more particularly, to a method and apparatus for megasonic cleaning of a semiconductor substrate following fabrication processes.
Megasonic cleaning is widely used in semiconductor manufacturing operations and can be employed in a batch cleaning process or a single wafer cleaning process. For a batch cleaning process, the vibrations of a megasonic transducer creates sonic pressure waves in the liquid of the cleaning tank which contains a batch of semiconductor substrates. A single wafer megasonic cleaning process often uses a relatively small transducer above a rotating wafer, wherein the transducer is scanned across the wafer using a liquid stream coupling, or in the case of full immersion in a single wafer tank system a larger transducer which can couple to a larger portion of the wafer. In each case, the primary particle removal mechanism from megasonic cleaning is by cavitation and acoustic streaming. Cavitation is the rapid forming and collapsing of microscopic bubbles in a liquid medium under the action of sonic agitation. Upon collapse, the bubbles release energy which assists in particle removal by breaking the various adhesion forces which cause the particles to adhere to the substrate. Sonic agitation involves subjecting the liquid to acoustic energy waves. Under megasonic cleaning, these acoustic waves occur at frequencies between 0.4 and 1.5 Megahertz (MHz), inclusive. Lower frequencies have been used for other cleaning applications in the ultrasonic range, but these applications are used primarly for part cleaning, and not semiconductor substrate cleaning, due to the potential for damage to the substrates at the lower frequencies.
FIG. 1A is a schematic diagram of a batch megasonic cleaning system. Tank 100 is filled with a cleaning solution. Wafer holder 102 includes a batch of wafers to be cleaned. Transducer 104 creates pressure waves through sonic energy with frequencies near 1 Megahertz. These pressure waves act in concert with the appropriate chemistry to control particle re-adhesion and provide the cleaning action. Because of the long cleaning times and chemical usage required for batch cleaning systems, efforts have been focused on single wafer cleaning systems in order to decrease chemical usage, increase wafer-to-wafer control, and decrease defects in accordance with the International Technology Roadmap for Semiconductors (ITRS) requirements. Batch systems suffer from another disadvantage in that the delivery of megasonic energy to the multiple wafers in the tank is non-uniform and can result in ‘hot spots’ due to constructive interference, or ‘cold spots’ due to destructive interference, each being caused by reflection of the megasonic waves from both the multiple wafers and from the megasonic tank walls. Therefore, a higher megasonic energy as well as multiple transducer arrays must be applied in order to reach all regions of the wafers in wafer holder 102. Single wafer megasonic which couple to the wafer through a meniscus also suffer from reflected power reducing the cleaning efficiency. FIG. 1B is a schematic diagram of a single wafer cleaning tank. Here, tank 106 is filled with a cleaning solution. Wafer 108 is submerged in the cleaning solution of tank 106. Transducer 110 supplies the energy to clean the wafer. One shortcoming of the single wafer cleaning tank is that particles remain inside the tank requiring that the cleaning fluid be replaced or re-circulated and filtered regularly. Furthermore, removal of the wafer from the tank after megasonic cleaning also runs the risk of particle reattachment.
FIG. 1C is a schematic diagram of nozzle-type megasonic cleaning configuration for a single wafer. Nozzle 112 provides fluid stream 114 through which the megasonic energy is coupled. Transducer 116, which is connected to power supply 118, provides the megasonic energy through the fluid stream 114 to the substrate as the fluid stream flows through the nozzle. Megasonic energy supplied through fluid stream 114 provides the cleaning mechanism to clean wafer 120. One shortcoming of the nozzle cleaning configuration includes requiring a high flow rate of fluid stream 114 to cool the transducer 116. Fluid stream 114 generated through nozzle 112 covers a small area, therefore, a fairly high megasonic energy is needed to clean the wafer in a reasonable time. The high energy required here necessitates cooling of the transducer. Consequently, the high flow rate of fluid stream 114 is due in good part to the cooling requirements, which are driven by the high energy requirements. This makes cleaning using a cleaning chemistry other than deionized water less desirable, due to cost associated with the high flow rates and effluent handling requirements.
In view of the foregoing, there is a need for a method and apparatus to provide a single wafer magasonic cleaning configuration that is capable of cooling the transducer or resonator while limiting the volume of cleaning chemistry consumed.